Line a cookie sheet with a silicone baking mat or parchment paper.
In the bowl of an electric mixer, using the paddle attachment, or in a large bowl using a hand mixer, cream the unsalted butter and the sugars on medium speed.
Add the egg and vanilla extract and beat on medium-high speed until combined.
Sift the dry ingredients (flour, baking soda and salt) into the wet ingredients.
Beat on low speed until combined.
Use a wooden spoon or rubber spatula to stir in the chocolate chips and marshmallows.
Use a medium-sized cookie scoop to scoop 1 1/3-tbsp cookie dough balls onto the prepared baking sheets, allowing at least 2 inches of space between cookies.
Bake 9-11 minutes until golden brown.
Cool on sheet pan for 5 minutes, then remove to wire rack to cool fully.
Notes
Cookies keep well in an airtight container for several days.